Coating a layer of one or more types of heterogeneous metal nanoparticles on a ceramic powder surface, especially on an oxide powder surface of aluminium oxide (Al2O3), zirconium oxide (ZrO2), silox (SiO2) and ferroferric oxide (Fe3O4) may make the ceramic powder possess the performances of a metal coating shell and a ceramic core at the same time, reduce a conglobation effect of the powder and segregation between each phase (especially metal and ceramics), give new performances to the oxide powder, such as a catalytic performance, an electromagnetic performance, or the like, or surface performances that effectively change the powder, such as an electrochemical performance and a diffusibility during sintering. Therefore, great attention is paid to the development of a coating technology and applications thereof in more fields including structure and function ceramics.
At present, preparation methods of metal-coated ceramic powder mainly include a mechanical mixing method, a sol-gel method, a chemical plating method, a chemical vapor deposition method, etc. Wherein, the mechanical mixing method is simplest, but it is difficult to uniformly mix powder with a larger difference on density property. The sol-gel method is a method to disperse materials in a solvate to form sol and gel through hydrolysis, and obtain a nanoparticle material required through drying and thermal treatment. Ni—Al2O3 powder had been obtained by Rodeghiero, et al, using the sol-gel method. Powder processed by sol and gel was subjected to thermal treatment in H2 gas environment for 1 h to reduce Ni salt into Ni, and then the powder was subjected to hot pressed sintering under a temperature condition ranging from 1350 to 1400° C. for 2-4 h, to obtain a dense Al2O3—Ni composite material (Materials Science and Engineering A, 1995, 195, pp 151-161). Compared with a solid phase reaction, the chemical reaction in the sol-gel method is easier to be conducted, and the composition temperature is lower, but the temperature of subsequent heating processing is usually higher (calcination, crystallization, and reduction to obtain metallic phase), which is easy to cause conglobation of the powder and slight sintering of the metallic phase.
The chemical plating is a widely applied method which coats powder through an electrochemical process without an extra electric field, and has the advantages of simple equipment, designable clad layer performance. Cao Xiaoguo, et al, used the chemical plating method to coat silver on a Fe3O4 powder surface in a water/ethanol medium using formaldehyde as a reductive agent. The test result showed that the silver layer uniformly and completely coated on the Fe3O4 powder surface effectively improved a powder agent electroconductibility of the Fe3O4 powder. (Material Engineering, 2007, 4, pp 57-60). Al2O3 powder coated by Ni was obtained by Mehmet Uysal, et al, using an electroplating process. Al2O3 powder was preprocessed in a SnCl2 solution firstly to improve a surface activity of the Al2O3 powder, and then uniformly distributed Ni nanoparticles were coated on the Al2O3 powder surface through controlling the pH value, NiCl2 concentration, and other technological parameters of the solution using NiCl2 as a Ni source (Ceramics International, 2013, 39, pp 5485-5493). However, the powder chemical plating has a certain particularity. The powder surface needs to have a good catalytic activity to implement uniform deposition of a modified layer on the surface, and necessary preprocessing needs to be conducted to activate the ceramics and other powder that do not have surface catalytic activity. Meanwhile, the plating solution needs to have a certain stability to avoid spontaneous decomposition so as to uniformly disperse the powder in the plating solution. Therefore, the application range thereof is limited by a certain extent.
The chemical vapor deposition method is to form a solid sediment through an aggregation reaction of raw gases on a particle surface, so as to implement a coating effect to the powder particle. A carbon layer was coated on a LiFePO4 powder surface with a particle size of 200 nm by Jiang Yong, et al, using the chemical vapor deposition method (Silicate Journal, 2008, 36, pp 1295-1299). Ni nanoparticles were coated on an aluminium oxide surface through pyrolysis by Zhang, et al, using the chemical vapor deposition method, and using a metal organic substance as a material, and heating the material to volatilize firstly, and then bringing the material into a high temperature reaction chamber through Ar. However, the nanoparticles are easy to conglobate and grow under a higher coating temperature, so as to reduce hardness and intensity of the powder after sintering (Journal of the European Ceramic Society, 2014, 34, pp 435-441).